1. Technical Field of the Invention
This invention relates to a sintering furnace for the production of a quartz preform which can be used for carrying out at least one of the steps of dehydration, fluorine-addition and sintering of a porous quartz soot preform prepared by a flame hydrolysis method such as a VAD method or OVD method.
2. Technical Background
As a furnace muffle material of a furnace for the production of a quartz preform such as a quartz preform for optical fibers, for example, there can be used high purity carbon having the inner wall and/or outer wall coated with a gas-impermeable material, shown in FIG. 2, as disclosed in Japanese Patent Laid-Open Publication No. 20163/1986. In FIGS. 2(a) and 2(b), a soot preform 1 is fitted to connection rod 2 which is rotatable and vertically movable in an electric furnace 3 provided with a heater of, e.g., carbon. Numeral 5 designates a furnace muffle of carbon inserted in the surface 3 and made up of parts 15-1 to 15-5. The surface of the tube is coated with SiC and further oxidized. Numeral 6 designates a gas feed port provided at the lower end of the furnace muffle 5 so as to feed a gas such as He, Cl.sub.2, SiF.sub.4, etc. into the furnace muffle 5 of the SiC-coated carbon. The coating of SiC on the surface of the furnace muffle 5 of carbon is carried out by an ordinary CVD method or plasma CVD method using SiCl.sub. 4, SiH.sub.4, SiHC1.sub.4, etc. as a raw material for the Si, and CH.sub.4 as a raw material for the C. Generally, the vapor deposition temperature is 1000.degree. C. in the case of the ordinary CVD method and 700.degree.-1000.degree. C. in the case of the plasma CVD method. The coating thickness of SiC is effectively at least 1 .mu.m when using it in an oxidized state. Furthermore, it is known that the furnace muffle of carbon is coated with surface-non-oxidized SiC or gas-impermeable carbon.
However, the above described technique has a problem in that a furnace muffle of SiC-coated high purity carbon, which is commercially available at the present time, has a maximum length of only about 900 mm and when a longer length is required, a plurality of the tubes must be joined. In the structure of the prior art apparatus as shown in FIG. 2(a), however, there is a joint at a relatively high temperature part inside or outside the furnace, except for a relatively small-sized apparatus. FIG. 2(b) is a partial cross-sectional view of such a joint, in which separate upper and lower muffle parts 15-i and 15-i+1 are respectively threaded to each other and the joint sealed by a carbon gasket 16.
However, the gas impermeable property of this structure is not complete, and measurement of the gas permeation from such a furnace muffle with an outer diameter of 210 mm showed 0.36 cc/min/mm.multidot.H.sub.2). Thus, a small amount of gas permeates through the furnace muffle so that O.sub.2 and H.sub.2 O outside the furnace muffle enters the furnace muffle and a problem arises that when a preform for optical fibers sintered in such a furnace muffle is finally converted into an optical fiber, the average value of transmission loss of the optical fiber becomes slightly larger than for a fiber prepared in an ordinary quartz muffle.
Since a quartz furnace muffle has a problem with respect to its life, in that in thermal deformation at a high temperature cracking occurs when the temperature is raised once and then lowered to 600.degree. C. or less, resulting in an increase in the cost. On the other hand, it has been necessary to develop a muffle formed of high purity carbon, capable of producing an optical fiber with an average loss substantially similar to a quartz furnace muffle.
The present invention has been made in the light of this situation and aims at providing a sintering furnace for producing a quartz preform, comprising a furnace muffle formed of high purity carbon coated with a gas-impermeable coating, and which is suitable for obtaining an optical fiber with a longer life as well as less loss due to impurities and OH groups.
The present invention has been made as a means for solving the above described problems, being an improvement of a method of sealing a joint of a furnace muffle of high purity carbon coated with a gas-impermeable film.